The relationship between acute lymphoblastic leukemia (ALL-L3) and stage IV small noncleaved cell lymphoma with bone marrow involvement (SNCL) was analyzed by retrospective study of 45 patients. Twelve patients with ALL-L3 had tumour restricted to the marrow and blood with no evidence of an extramedullary tumor at their initial presentation. Nineteen patients with stage IV SNCL had evidence of extramedullary disease in addition to marrow and blood involvement. Fourteen patients with marrow and blood involvement by SNCL at the time of a first or second relapse were also studied. The median age of the ALL-L3 patients was 33.5 years, compared with 13 years for the patients with stage IV SNCL and 10.5 years for the marrow relapse patients. All of the ALL-L3 patients wre male; the male:female ratio for the SNCL patients was 17:2; for the relapse group, 9:5. The majority of the ALL-L3 patients had symptoms related to bone marrow failure; the majority of the SNCL patients had symptoms related to a tumor mass. Laboratory values at the time of initial diagnosis were similar for the ALL-L3 and SNCL patients, except that the median platelet count was lower in the ALL-L3 group. Despite extensive marrow involvement by tumor in most patients, frankly leukemic blood pictures were uncommon. Light-chain-restricted surface immunoglobulin on tumor cells was a consistent finding in all cases tested; the findings were similar in the three groups. Cytogenetic data, available for 19 cases, showed no difference between cases with and without extramedullary disease. The median survival was 5.1 months for ALL-L3 patients and 3.2 months for SNCL patients. The median survival from the time of bone marrow relapse was 2.3 months. These observations suggest that ALL-L3 and SNCL represent different manifestations of the same disease process.